How many calories do I burn a day?

The resting half of that calculation comes from a published regression. Mifflin-St Jeor gives 10 times weight in kilograms, plus 6.25 times height in centimetres, minus 5 times age, then plus 5 for men or minus 161 for women. For the reference body above: 800 + 1,125 - 150 + 5 = 1,780 kcal a day.

The other half is an activity multiplier, and it is worth knowing that this ladder - 1.2, 1.375, 1.55, 1.725, 1.9 - has no primary source. It is a field convention. The published counterpart is the FAO/WHO/UNU physical activity level framework, which documents bands of 1.40 to 1.69, 1.70 to 1.99 and 2.00 to 2.40 for adult lifestyles; the bottom two rungs of the conventional ladder fall below all of them.

The band matters more than the point estimate. Frankenfield's 2005 review judged resting equations by whether they landed within 10 percent of a measured value; Mifflin-St Jeor did so more often than the alternatives, and missed by more for some people. That 10 percent turns 1,780 into 1,602 to 1,958, and at a 1.55 multiplier turns 2,759 into 2,483 to 3,035 - a 552 kcal span.

None of this is measured. Total expenditure is measured with doubly labelled water; a web form evaluates a line fitted through other people. The practical check is to hold intake steady for two or three weeks and watch the weight trend, which measures the one body the equations were averaging over. See how TDEE is calculated for the full derivation, or why TDEE calculators disagree for where the differences come from.